Obligatory mark for select options

Hi experts,

I have a requirement where both low and high field of select -option should be mandatory.

Code i have written is :s_period for cgpl_project-createdon.

But only the low field is made obligatory.How to make high field as oblogatory too....

but if we write abap code manually it will work..

if s_period is intial..

error mesg like " date is mandatory''

but user want that obligatory mark only..how can i acheive this....

can any body plz help me..

Regards,

Praveena..